The invention relates to a method for reducing metals and more particularly, to a method of reducing and precipitating a group of nanostructured metal materials by tin and antimony porphyrins.
A porphyrin is a cyclic tetrapyrrolic system consisting of a 20-carbon skeleton and has been used in a variety of electrical, optical, structural, and catalytic applications. Metal ions can covalently bond within the porphyrin structure. Various peripheral groups, both inorganic and organic, can be attached to the 20-carbon skeleton to provide desired physical, chemical, and optical characteristics.
Metalloporphyrin complexes exhibit a wide range of biological functions in proteins. For instance, the iron porphyrin (heme) in cytochrome c3, a well studied protein found in iron-reducing bacteria (Shewanella putrefaciens) or sulfate-reducing bacteria (e.g., Desulfovibrio vulgaris) is likely involved in electron transport. Recently, it was recognized that cytochrome C3 also catalyzes the non-biological reduction of metals such as U, Cr and Se.
Photoinduced redox reactions of a three-component system containing a photosensitizer, an electron donor, and an electron acceptor have been studied by several authors. Metallo-porphyrins are well-studied photosensitizers for the reduction of various acceptor molecules, usually methylviologen. For example, photoreduced tin porphyrins act as strong reductants in solution, in micelles, and at water-organic solvent interfaces, upon excitation by visible light and reduction by an electron donor such as a tertiary amine.
Shelnutt (Shelnutt, J., J. Amer. Chem. Soc., 1983, 105, 7179-7180; U.S. Pat. No. 4,568,435, issued on Feb. 4, 1986; both herein incorporated by reference) studied the ternary system comprised of Sn(IV) protoporphyrin IX (SnPP), TEA, and methylviologen (MV2+), where SnPP is the photosensitizer, TEA is the electron donor, and MV2+ is the electron acceptor. The photoinduced oxidation-reduction reaction is illustrated in FIG. 1. Irradiation of SnPP by visible light leads to excitation of the porphyrin to its lowest-lying triplet xcfx80xe2x80x94xcfx80 state (SnPP*). Because the redox potential of the couple SnPP*/SnPP (+1.1 V) is higher than that of TEA/TEAox (+0.82 V), excited SnPP* accepts an electron from TEA resulting in the radical porphyrin anion (FIG. 1). The low redox potential of the SnPPxe2x88x92/SnPP couple (xe2x88x920.66 V) allows the reduction of MV2+ to MV+ (MV2+/MV+; xe2x88x920.45 V).11 The quantum yield for the reaction is near 0.8. This photochemical cycle is reductive, a feature that is to-date unique to Sn(IV) and Sb(V) porphyrins. What makes the cycle reductive is that reduction of the porphyrin, rather than oxidation, is the initial step following photoexcitation. Several metalloporphyrins such as Zn porphyrins are known to follow an oxidative cycle.